Patent GB-1180487-A (GB)
Title: Improvements in or relating to Telephone Systems
Inventor: CROOKS STEPHEN JOHN
IPC: H04M9/02
Filed: 1967-06-02
Abstract: 1,180,487. Interconnection systems. GENERAL ELECTRIC & ENGLISH ELECTRIC COMPANIES Ltd. 30 Aug., 1968 [2 June, 19671, No. 25509/67. Heading H4K. In a TDM party line system in which there are a greater number of substations than channels in the multiplex cycle, each substation is allotted a characterizing signal which is transmitted by the calling party in a free time slot and the called substation on recognizing its own signal synchronizes its own operation at this time slot. The system is concerned with eliminating crosstalk due to transit time delays in the highway. First embodiment.-A pulse generator is connected to the common highway and supplies a small-amplitude timing pulse in each channel time slot and an opposite polarity marking pulse once in each multiplex frame. Each substation is identified by a particular audio frequency to which its key set is tuned when the station is on-hook. When a substation calls he sets his key set to the wanted party's " number " so that the marking pulse is modulated with the relevant frequency and at the same time a counter and associated logic is started so as to hunt for a free time slot. The calling frequency is then also modulated on the pulse in this time slot. At the wanted party's set, reception of the calling frequency at the marking time slot starts a counter and associated logic so as to find the calling frequency in the " free " time slot to which the set then becomes locked. An incoming speech amplified is enabled to provide a ringing signal. At the end of the call both sets are restored to their rest conditions when either party hangs up. Second embodiment (Fig. 2, not shown).-A pulse generator supplies narrow timing pulses (but not marker pulses) in each time slot to the common highway. As before each substation is identified by a characteristic frequency. When a subscriber calls, logic circuitry in his set hunts for a free time slot and then transmits the called party's frequency in this slot as modulation on a pulse of opposite polarity to that of the timing pulse which also occupies a small portion of the slot. In the called party's set, scanning of the channels is stopped upon recognition of its own characteristic frequency and a calling tone is generated so as to alert the called party and also to inform the calling party. At the end of the call the sets are restored to their scanning condition on hang-up at either end. - Third embodiment, separate speech and signalling highways (Figs. 3 and 4, not shown). A pulse generator supplies a narrow positive timing pulse for each time slot on the speech highway and applies a large positive marker pulse followed at intervals of one frame by small positive line pulses to the signalling highway. One line pulse is allotted to each substation which is thereby identified by the time position of the line pulse relative to the preceding marker pulse. A calling subscriber chooses a free time blot and then transmits a negative pulse in this time slot (on both highways) during the frames allotted to the called party. The latter's set (assuming it is free) is locked to its characterizing frame and therefore responds to the negative pulse occurring in this frame so as to transmit a calling tone to the subscriber. The call then proceeds using the selected time slot in each frame. Release is under the control of either party. Fourth embodiment, common speech and signalling highways. Herein the marker pulse is denoted by omission of a line pulse while the calling and line pulses are increased in amplitude to further distinguish them from the timing pulses.